During the drilling phase of an oil or gas well, it is necessary to ensure the wellbore pressure integrity is maintained at all times. This necessity arises because it is customary to provide a well drilling fluid that is passed downward through the drill string and upward external to the drill string in order to cool and lubricate the drill bit, as well as carry away the cuttings produced by the drill bit. The drilling fluid, also known as mud, maintains hydrostatic pressure on the subterranean zones through which the wellbore is drilled and circulates cuttings out of the wellbore. It also, under ideal conditions, creates an impermeable filter cake along the walls of the wellbore that prevents loss of the drilling fluid, maintains wellbore wall integrity (i.e. prevents cave-ins), and minimizes formation damage due to drilling fluid invasion. Subterranean vugs, fractures and other thief zones are often encountered during drilling whereby the drilling fluid circulation is lost, and drilling operations must be terminated while remedial steps are taken.
In addition to underground blowouts, cross flow, and loss of hydrostatic pressure, lost circulation can lead to a drill pipe becoming lodged in the wellbore. Some formations are very porous, so that a considerable flow of drilling fluid can be forced into the rock. (Some “vuggy” formations may even contain natural cavities.) In extreme circumstances, from tens to hundreds of barrels of drilling fluid can be forced into the rock, which can often cause permanent fractures. In these extreme cases and in other severe situations involving vugs, fractures, formation cavities and the like, placing a high yield point material similar to the consistency of window caulking is a viable option to plug off the zone. Although commercial products like this exist, a method to accurately predict the mixing energy required to optimize these products was not previously identified prior to the present invention. See Gockel, J. F., et al., “Lost Circulation: A Solution Based on the Problem”, presented at 1987 Society of Petroleum Engineers/International Association of Drilling Contractors (SPE/IADC) Drilling Conference, New Orleans, La., Mar. 15-18, 1987. (SPE Paper No. 16082) Canson, B. E., “Lost Circulation: Treatments for Naturally Fractured, Vugular or Cavernous Formations”, presented at the SPE/IADC 1985 Drilling Conference, New Orleans, La., Mar. 6-8, 1985. Sanders, W. W., “Lost Circulation: Assessment and Planning Program: Evolving Strategy to Control Severe Losses in Deepwater Projects”, presented at the SPE/IADC Drilling Conference, Amsterdam, The Netherlands, Feb. 19-21, 2003. (SPE paper No. 79836). All of the above are hereby incorporated by reference.
While a variety of compositions have been developed and used for combating lost circulation, cross flows and underground blowout problems, such compositions have often been unsuccessful due to delayed and inadequate viscosity development by the compositions. An appreciable yield point and a significant level of viscosity, or the degree to which a fluid resists flow under an applied force, is needed in order for the compositions to combat the aforementioned lost circulation. For example, a variety of cement compositions have been used in attempts to stop lost circulation. The lost circulation is usually the result of encountering weak subterranean zones that contain natural fractures or are fractured by drilling fluid pressures and rapidly break down. U.S. Pat. No. 1,807,082 issued May 26, 1931, to Boynton discusses the introduction of mica flakes into the well fluid circulation for coating the wall of the wellbore. U.S. Pat. No. 2,342,588 issued Feb. 22, 1944, to Larkin discloses the method of mixing a quantity of small pieces of sponge rubber with the well drilling fluid. The sponge rubber particles are deposited in the cracks and fissures and thereafter expand to fill them. U.S. Pat. No. 2,353,372 issued Jul. 11, 1944, to Stone discloses the mixing of fragmented organic grain less foil with the well drilling fluid for circulation therewith and disposition within the cracks and fissures of the wellbore walls for reducing the lost circulation of the well drilling fluid. U.S. Pat. No. 2,634,236 issued Jul. 14, 1953 to Fisher discloses the admixing of fiberized leather with the drilling fluid. U.S. Pat. No. 3,221,825 issued Dec. 7, 1965, to Henderson discloses the mixing of cork particles with the well drilling fluid for sealing off the cracks and fissures of the wellbore walls. U.S. Pat. No. 3,254,064 issued May 31, 1966, to Nevins discloses the use of solid, stretchable, deformable organic polymers in the well drilling fluid for blocking off leaks in the wellbore walls. U.S. Pat. No. 3,568,782 issued Mar. 9, 1971, to Cox discloses the use of popcorn in the well drilling fluid. U.S. Pat. No. 3,788,405 issued Jan. 29, 1974, to Taylor discloses the use of a mixture of straw and chemical wood pulp fibers for blocking off the lost circulation in the wellbore. U.S. Pat. No. 4,222,444 issued Sep. 16, 1980, to Hamilton discloses using magnetic material, such as discarded magnetic tape, to block the unwanted loss of fluid in a wellbore. U.S. Pat. No. 6,060,434 issued on May 9, 2000, discloses using oil-based compositions for sealing subterranean zones. U.S. Pat. No. 6,258,757 issued on Jul. 10, 2001 to Sweatman discloses using water based compositions for sealing subterranean zones. All of the above are hereby incorporated by reference.
Solutions, such as the ones found in the 6,060,434 and 6,258,757 patents, often use two streams of materials to combat lost circulation problems. For example, drilling mud and reactant FlexPlug®, commercially available from Halliburton, can be used downhole to form a highly viscous paste-type material with the consistency of window caulking. It has been found in the present invention that the ability of FlexPlug® to withstand wellbore pressures and combat lost circulation depends upon the chemical formulation of the reactants, the mass ratio of wellbore fluids to product slurry(s), and the degree of mixing. The degree of mixing can be generally quantified in terms of mixing energy (such as Joules/Kg, etc.). An increase in the mixing energy usually yields a higher quality product.
There are different chemical recipes that can be used as downhole reactants. The term “chemical recipe” is generally used to refer to the contents of the chemical treatment. Therefore, the chemical recipe is the mix of chemicals that the designer uses to combat lost circulation.
The chemical recipe may be water or oil based. In a water based chemical recipe, the compositions and methods are particularly suitable for sealing subterranean zones containing oil based drilling fluids, e.g., water in oil emulsions, known as inverted emulsions. The compositions are basically comprised of water, an aqueous rubber latex, an organophilic clay, and sodium carbonate. The compositions can also include one or more latex stabilizers, dispersing agents, biopolymers, defoaming agents, foaming agents, emulsion breakers, fillers, rubber vulcanizing agents and the like.
The second type of chemical recipe is the oil-based recipe. The compositions are basically comprised of oil, a hydra table polymer, an organophilic clay, and a water swellable clay. The compositions can also include cross-linking agents, dispersing agents, cement, fillers and the like. When the sealing compositions of this chemical recipe contact water in the wellbore, the hydra table polymer reacts with the water whereby it is hydrated and forms a highly viscous gel, and the water swellable clay swells whereby an ultra high viscosity mass is formed.
These chemical recipes are generally delivered to a downhole wellbore as one stream, mixing with a second or more streams of wellbore fluids at the desired downhole location. The composition and mixing of the recipe with the wellbore fluids dictate the quality of the product of the mixture. For a dual stream reaction between FlexPlug® and drilling mud, it has been found that the preferred volumetric ratio is 1:1 for most drilling muds encountered, but is not limited to 1:1 ratio.
Historically, the rate at which these reactive products have been pumped and placed has been based on rules of thumb or surface equipment limitations, but no consideration has been taken for the effect of this rate on the quality of the final product. This lack of consideration of mixing energy during the placement of a multi-stream reactive product has been the result of the lack of accurate modeling and scaling techniques of the mixing phenomena (energies and macromolecular interactions) of multi-stream chemical treatments, resulting in the lack of empirical data to prove the importance of mixing energy. There is no current technology that can provide accurate guidance to the proper design of multi-stream chemical treatments. No models or systems have been capable of taking the myriad of variables present in downhole conditions and combine them in a way to accurately predict the required mixing energy for a chemical recipe. The result of this problem is sometimes a failure to cure the loss zone, which may have been avoided had a procedure backed by recommendations from modeling been available.
There are several categories of variables that can be adjusted at the drill site. First, the materials, chemicals, and design of the drill string may be adjusted to particular well conditions. Second, the flow rate and pressure of the substances being pumped into the wellbore may be adjusted. The present invention suggests a way to optimize the mixing energy of a multi-stream treatment by manipulating the variables mentioned above. This is in part because the mixing phenomenon (energies and macromolecular interactions) of chemical treatments have never been accurately modeled or scaled.